Classical conditioning occurs when a stimulus is associated with a consequence (reward or punishment).

This was shown by Ivan Pavlov with dogs.

An unconditioned stimulus (UCS) elicits a specific, unconditioned response (UCR).

If the UCS is associated with a neutral stimulus (NS), the latter may become a conditioned stimulus (CS) and elicit the same, conditioned response (CR), in the absence of the UCS.

Advertisers often use classical conditioning to associate their products (NS) with a desirable stimulus (UCS).